The light forming a real-world image which is focused onto an imaging device has an inherent efficiency for detection. The photon flux through an aperture falls onto the sensor surface and is converted into electrical signals which are ultimately stored as digital images. Each pixel of the stored image is typically composed of signals from three individual photodetectors corresponding to the three fundamental colors of the red, green, blue (RGB) domain, or the complimentary cyan, magenta, yellow (CMY) domain. The intensity of the electrical signal is roughly proportional to the quantity of photons impinging each photodetector. A lens, or microlens, is typically used to focus the incoming light onto the active area of each photodetector. The microlens may be formed by any set of process steps which generate a convex shaped lens having a high refractive index, e.g., >1.5, material located between the light source and the photodetector.
Typical lens formation processes include resist reflow or dielectric material reflow techniques. After the reflow process, a hemispherical profile having a circular footprint results. The fill factor, defined as the ratio of lens to cell areas, for a circular footprint, when just touching neighboring cells, is about 79%.
U.S. Pat. No. 5,324,623, to Tsumori, granted Jun. 28, 1994, for Microlens forming method, describes a method of forming a microlens made of a thermoplastic resin on a solid-state imaging device, and more particularly to a microlens forming method which can form a microlens made of a thermoplastic resin on a solid-state imaging device which is highly accurate, having a fine pattern of 0.5 μm or less.
U.S. Pat. No. 6,163,407, to Okazaki et al., granted Dec. 19, 2000, for Microlens array and method of forming same and solid-state image pickup device and method of manufacturing same, describes a microlens array in which a plurality of microlenses are arranged, a method of forming the microlens array, a solid-state image pickup device having the microlens array, and a method of manufacturing the solid-state image pickup device.
U.S. Pat. No. 6,417,022, to Hsiao et al., granted Jul. 9, 2002, for Method for making long focal length micro-lens for color filters, describes a method for fabricating micro-lenses for color filters and devices, and more particularly, relates to a method for fabricating long focal length micro-lens for color filters and devices fabricated by such a method.
U.S. Pat. No. 6,495,813, to Fan et al., granted Dec. 17, 2002, for Multi-microlens design for semiconductor imaging devices to increase light collection efficiency in the color filter process, describes optical design and microelectronic fabrication of microlens arrays for the optimization of spectral collection efficiency and related photodetector signal contrast in the color filter process of semiconductor imaging devices.